The heating of hot-rolling cylinders is due to the transmission of heat to the rolls by conduction from the product, such as a strip of metal, that is being rolled. In recent years, the cooling of rolling cylinders has been intensively studied because of its very large impact on the deterioration of said cylinders (wear) as a result of the thermomechanical fatigue generated and on the control of the curve of the cylinders. The deterioration of the cylinders has a very great impact on the quality of the product.
A typical installation for cooling work cylinders in a rolling stand is for example described in documents JP-A-2001 340908, JP-A-2001 001017, JP-A-07 116714, JP-A-05 104114, JP-A-63 39712, JP-A-61 176411 etc. Cooling-water tubes, modules or tanks are equipped with atomisers and positioned around each cylinder, with a means for supplying cooling water. Guide plates for the cooling water are positioned in association to the upper cylinder and to the lower cylinder. These plates are equipped with a scraper, for example covered with rubber, associated to each of the cylinders in order to prevent the water from flowing over the product that is being rolled.
A major problem to be solved in the case of the cooling of work cylinders is that of obtaining homogeneous cooling across the width and around the circumference. Solutions exist in which the flows supplied by the various nozzles of a cooling module are individually regulated on the basis of data provided by a sensor, such as an infrared thermometer (for example JP-A-12 24105). Another solution consists in using heads with water-spraying holes distributed according to an appropriate pattern, in the axial dimension and in the dimension of the circumference (JP-A-10 291011). A third solution is to use a motorised head with nozzles on side guides (EP-A-0 599 277).
Recent authors recognise for one thing that the impact of the nozzles positioned as close as possible to the rollgap turns out more effective and for another that intensive cooling by flat nozzles has a reduced impact on the temperature of the roll than the surface covered (YE, X. and SAMAVASEKARA, I. V., The Role of Spray Cooling on Thermal Behaviour and Crown Development in Hot Strip Mill Work Rolls, Transactions of the ISS, July 1994, p. 49). One possible consequence of the application of cooling of the roll close to the point of exit from the roll is an increase in the tension gradient on the surface of the roll and a worsening of the cracking (“fire crazing”), but with a lower temperature below the surface of the roll (SEKIMOTO et al, SEAISI Quarterly, April 1977, p. 48).
It is known that the type of spray (or nozzle) used for cooling rolls has a significant effect on the HTC values. VAN STEDEN and TELLMAN in A new method of designing a work roll cooling system for improved productivity and strip quality, Fourth International Hot Rolling Conference, Deauville, France, 1987, compared the performance of nozzles with flat, square and oval jets by measuring the thermal response of a plate attached to a cylinder after heating to 400° C., followed by cooling by water atomisation when the cylinder is rotated. Values of up to 140 kW/m2.K were obtained for the range of nozzles considered. This work showed that the highest HTC value relative to the atomising peak is achieved by the nozzle with the flat type of jet. However, this study obviously ignores the fact that the same cooling performances may be obtained by a nozzle with a lower peak HTC value but whose jet is applied over a much greater part of the surface of the roll. One therefore notes significant differences in the literature concerning both the HTC value associated with the nozzle and the suitability of various types of nozzles for the effective cooling of rolls.
It is certain that, in the rolling of flat strips, cooling systems based on nozzles with flat jets can be further improved. However, these improvements are limited and the costs are very high since one is working at high pressures and high flow speeds.
In recent years, various alternative cooling technologies have been patented based on heads positioned close to the surface of the work cylinder and with a flow circulation (for example EP-A-919297, JP-A-11 033610). However, no industrial applications of these cooling systems are known. Roll-cooling devices are thus also known in which a cooling head is shaped to ensure that the water is guided over the surface of the roll. The surface of the head is separated from that of the roll by a gap in which the cooling water circulates, creating a sort of “sleeve” (JP-A-61 266110, JP-A-63 303609, JP-A-20 84205). The water may either be fed through one end of the head and drained at the other end (JP-A-20 84205) or be fed through both ends and drained at the centre (EP-A-919 297), the draining occurring through the head itself, with scraper systems preventing leakage around the circumference of the rolls. Draining to the outside may also occur between one end of the head and the surface of the roll (JP-A-11 277113). Document JP-A-58 047502 describes moreover a cooling shoe that is deformable by means of springs so as to adapt to the surface of the roll.
In these systems, there are no water-supply atomisers distributed over the whole surface of the cooling head but instead, there is generally one single atomiser.
The Applicant began to examine alternative cooling technologies in 1993. Trials were conducted with a cooling head in a high turbulence, low-pressure (HTLP) environment and with a water pillow cooling (WPC) head positioned beyond the scraper. Both technologies allow to create strong turbulence on the surface of the roll. In this way, a very homogeneous cooling pattern is obtained. Preliminary simulations of highly turbulent cooling have shown the potential of this technology for cooling work cylinders. Highly turbulent cooling reduces thermal fatigue and hence deterioration of the surface of the work cylinder. Moreover, for the same flow of heat dissipated during cooling, this technology requires lower flow speed and pressure compared with traditional configurations for cooling by vaporisation with a flat jet.